Structures based on heterojunctions of AlGaN and GaN are generally used as electronic devices containing a nitride semiconductor.
A particular structure includes a buffer layer which is placed on a sapphire or Si substrate and which is made of a nitride semiconductor, a channel layer which is placed on the buffer layer and which is generally made of GaN, a barrier layer which is placed on the GaN channel layer and which is made of AlGaN, a source electrode, a drain electrode, and a gate electrode placed between the source and drain electrodes. The source and drain electrodes form an ohmic contact with a two-dimensional electron gas region formed at the interface between the AlGaN barrier layer and the GaN channel layer.
In the case where a nitride semiconductor is formed on a sapphire or SiC substrate, there is not a very serious problem. In the case of using a Si substrate having a thermal expansion coefficient less than that of a nitride semiconductor, the Si substrate warps to form a downwardly convex curve after the growth of a nitride semiconductor layer and cracks are formed in crystals by stress. Therefore, the Si substrate is not suitable to fabricate an electronic device.
A technique for reducing the difference in thermal expansion coefficient between a Si substrate and a nitride semiconductor is a “semiconductor electronic device” disclosed in Japanese Unexamined Patent Application Publication No. 2005-85852 (Patent Literature 1). In the semiconductor electronic device, a buffer layer, a GaN electron travel layer (500 nm), an AlGaN electron supply layer (20 nm), and a GaN contact layer are stacked on a GaN intervening layer formed on a silicon substrate. The buffer layer is composed of one or more first layers made of GaN and one or more second layers made of AlGaN, the first and second layers being alternately stacked in that order. Since the buffer layer, which is composed of the first and second layers different in material, is interposed, the direction of dislocation defects propagating from a lower side is bent and therefore the propagation of the dislocation defects in a growth direction is suppressed.
However, the semiconductor electronic device disclosed in Patent Literature 1 has a problem below.
A mechanism to form a two-dimensional electron gas in a nitride semiconductor is shown in FIG. 6. As shown in FIG. 6, an AlGaN layer (the AlGaN electron supply layer described in Patent Literature 1) having a thickness insufficient to cause stress relaxation and a small lattice constant is placed on a GaN layer (the GaN electron travel layer described in Patent Literature 1) which is stress-relieved and which has substantially a bulk lattice constant. In this case, piezoelectric polarization Ppe is induced by the difference in spontaneous polarization Psp between the GaN layer and the AlGaN layer and the fact that the AlGaN layer on the GaN layer is strained in-plane by stress +σ. As a result, the two-dimensional electron gas (2DEG) is formed at the interface therebetween.
In the case where a buffer layer (the GaN (Al composition=0)/AlGaN (1≧Al composition>0) buffer layer described in Patent Literature 1) formed by alternately growing AlGaN layers having different Al compositions as shown in FIG. 7 is considered as an average block, the buffer layer can be considered to be equivalent to a stress-relieved AlGaN layer by the same principle. Thus, a GaN layer (the GaN electron travel layer described in Patent Literature 1) formed on the stress-relieved AlGaN layer has a lattice constant larger than that of the AlGaN layer and therefore is strained by stress −σ in contrast to the case shown in FIG. 6 and a two-dimensional hole gas (2DHG) is formed at the interface therebetween.
There is a problem in that a two-dimensional hole gas formed in the electronic device as described above causes a leakage current to reduce device properties.